The combination of geostatistics and geochemical simulation for the site-specific management of soil salinity and sodicity

• Clay and silt contents showed a moderate spatial dependency.
• Soil chemical properties were strongly spatially dependent.
• Changes in the chemical quality of a saline-sodic soil were simulated by PHREEQC.
• The ESP remained unchanged after long-term irrigation with saline-sodic water.
• The cultivation of sugar beet was not significant in reducing soil sodicity.

The spatial variability of soil parameters such as salinity and sodicity is very effective in determining the suitable areal cropping pattern and appropriate management of agricultural lands. To conduct this study, a grid sampling at 100 m was carried out in an agricultural area of 30 ha located in Tajarak of Hamedan, western Iran. After physicochemical analyses of soil samples, soil properties which had significant correlations with the exchangeable sodium percentage (ESP) were identified using the statistical analysis. The ESP was significantly positively correlated with the clay content, pH, electrical conductivity (EC), cation exchange capacity (CEC), and sodium adsorption ratio (SAR), while it showed a significant negative correlation with the silt content. The spatial variability of ESP and its related parameters investigated by the geostatistical analysis showed that pH, EC, CEC, SAR, and ESP were strongly spatially dependent, while the clay and silt contents had a moderate spatial dependency. The distribution maps indicated that most soils in the study area did not have a sodicity problem, while ESP of soils in some parts was more than 15%. The long-term impact of the application of saline–sodic and non-saline–sodic irrigation waters on some chemical properties of a saline sodic soil under 20 years of the continuous cultivation of wheat and alternate cultivation of wheat and sugar beet was simulated using the geochemical PHREEQC program. The results showed that the chemical quality of the soil was significantly improved after 20 years of irrigation with non-saline–sodic water. On the other hand, the problem of the soil salinity was accelerated after 20 years of irrigation with saline–sodic water, while ESP remained unchanged. Due to the high water requirement of sugar beet, the effect of its cultivation was not significant in reducing soil sodicity despite the removal of Na+ from the soil by this crop. Therefore, it is recommended that the cropping pattern should be changed by planting crops with low water requirements or rainfed crops if sodic irrigation water is applied.